Photometric detection of internal gravity waves in upper main-sequence stars. IV. Comparable stochastic low-frequency variability in SMC, LMC, and Galactic massive stars

Massive main-sequence stars have convective cores and radiative envelopes, but also sub-surface convection zones caused by partial ionisation. However, the convective properties depend on opacity and a star's metallicity. Non-rotating 1D evolution models of main-sequence stars with the metallicity of the SMC suggest tenuous sub-surface convection zones using the Rayleigh number as a criterion for convection owing to their lower metallicity. We test whether massive stars of different metallicities both inside and outside of asteroseismically calibrated stability windows for sub-surface convection exhibit different properties in stochastic low-frequency (SLF) variability. We extracted customised light curves from the TESS mission for a sample of massive stars using an effective point spread function (ePSF) method, and compared their morphologies in terms of characteristic frequency and amplitude using a Gaussian process (GP) regression methodology. We demonstrate that the properties of SLF variability are generally consistent across the metallicity range from the Milky Way down to the SMC, for stars both inside and outside of the sub-surface stability windows. We conclude that non-rotating 1D stellar structure models cannot alone be used to explain SLF variability in light curves of massive stars. The similar properties of SLF variability across a range of metallicity values, which follow the same trends in mass and age in the HR diagram at both high and low metallicity, support a transition in the dominant mechanism causing SLF variability from younger to more evolved stars. Specifically, core-excited internal gravity waves (IGWs) are favoured for younger stars lacking sub-surface convection zones, especially at low metallicity, and sub-surface convection zones are favoured for more evolved massive stars. (abstract abridged for arXiv)